The satellite RNA of tobacco ringspot virus, abbreviated STobRV RNA, has startling chemical and biological properties. When it is coinoculated with tobacco ringspot virus, STobRV RNA reduces the extent of virus replication and protects the plant from the most severe effects of the virus. STobRV RNA occurs in repetitive sequence, multimeric forms as well as the 359 nucleotide residue "monomeric" form. Dimeric and trimeric forms of the RNA from virus particles spontaneously cleave to produce authentic, biologically active monomeric STobRV RNA in a reaction that may reflect a part of the replication cycle in vivo. This autolytic processing reaction clearly is non-enzymic beacuse of its resistance to proteinase treatments and because in vitro transcripts of cloned STobRV RNA sequences also spontaneously cleave. The reaction occurs even with RNAs that have most of the STobRV RNA sequence deleted. We will determine what is the minimum, or close to the minimum, polyribonucleotide chain that is sufficient for autolysis. From the corresponding cloned sequences in plasmids we will produce abundant in vitro transcripts. The two RNA products of the autolytic reaction will be probed, both separately and together, with structure sensitive endonucleases. We will attempt to grow crystals of the RNAs, separately and together, for future X-ray diffraction studies. Finally, the nucleotide sequences will be altered to determine their contributions to the autolytic reaction. We will examine both naturally occurring nucleotide substitutions, from STobRV RNA strains, an induced mutations. Various selection systems for mutants are proposed. Although no strict analogue of the plant virus small satellite RNAs has been discovered in association with animal or human viruses, we believe that the satellite RNA phenomenon deserves attention because of its potential for understanding how virus increase can be controlled. Autolytic RNA demands investigation not only on these grounds but also bacause of its unusual chemical and biological properties.